Filament winding is a well-known process that has been used in various industries to manufacture products having high structural efficiency in terms of strength and stiffness. Filament winding generally involves winding a fiber bundle that is impregnated with a thermosetting or, less generally, a thermoplastic resin matrix onto a suitably shaped mandrel or mold. Frequently, in this process, the mandrel is a body of revolution, but this does not have to be the case. The fiber bundle typically referred to as “tow” in the case of carbon or graphite or “roving” in the case of glass, is applied to the mandrel under tension. During filament winding, tension is applied to both maintain fiber collimation and to create radial stress (“prestress”) in the component being wound. Once the mandrel is covered to the desired thickness, the mandrel is often heated in an oven autoclave to cure or set the resin. Following curing, the mandrel may be removed from the wound fiber and a hollow, high-efficiency structure remains. Alternatively, the prestressed fiber is left in place to provide a prestressed layer on the mandrel. Such is the case when the mandrel forms a rail gun or a gun barrel.
Prestressing has been used for centuries dating back to times of the Napoleonic Wars when wire was wrapped under tension onto cast cannon barrels to improve barrel life. This process generally was referred to as “autofrettaging” for metallic structures. Prestressing materials this way is known as an important process for manufacturing parts requiring substantial fatigue strength and structural integrity. Today, prestressing is used in a variety of industrial and military applications.
Wet filament winding is one type of filament winding. In wet filament winding, a thermoset resin is impregnated into dry fibers during the filament winding operation. However, during this process, radial stress created by fiber tension causes the resin to flow or migrate. When the resin migrates this way, tension begins to be lost. This problem is exacerbated during curing (polymerization) under heat, which typically causes a reduction in the viscosity of the resin.
In some instances, to prevent loss of tension, the curing is carried-out in stages in which layers were successively cured. However, staged winding generally requires time intensive manufacture.
Alternatively, in some instances, the use of a thermoplastic matrix in filament winding has been utilized to achieve the ‘locking-in’ of the applied fiber tension. This is typically done by instantaneously cooling-down the pre-heated resin ‘in situ’, as the fibers are being laid down. However, using such materials is generally unsuited for many applications, involving relatively high material costs and complex processing equipment.
The use of a thermosetting resin and ‘in situ’ curing using a combination of fast reacting resin matrix accelerators and the application of heat has also been proposed for continuously curing during the filament winding process. While high prestress components can be produced in this manner, the process is relatively difficult to carry out, requires cumbersome equipment and machines, does not provide much flexibility in operation, and utilizes a cure that is not truly “instant” and therefore permits some undesirable resin migration and lost tension.
It would be advantageous to improve upon present processes to enhance the functionality, reliability and safety associated with use of filament wound devices that require radial prestress to be present. Therefore, what is needed is a filament winding process which allows for an easily implemented and cost effective means for maintaining significant radial prestress by suppressing resin flow and the consequent loss of tension and radial pressure.